More major structural problems have been discovered in the 200-year-old building that will drive up the cost of repairs and upgrades from $123.8 million to just under $150 million, The Post has learned.

Mayor Bloomberg authorized raising the repair budget by another $25 million about two weeks ago after being told that concrete in the roof deck of the rotunda was crumbling and the wire mesh supporting it was severely corroded at several locations.

“Like any homeowner, he was not particularly thrilled to hear that,” said a source.

The mayor gave the green light to replacing the failing concrete, a task that will have to be done manually in confined quarters because interim patch jobs weren’t holding, the source added.

The job is so difficult that it may not be completed until the end of Bloomberg’s administration in December 2013.

A reporter who toured the roof area above the building’s third floor was able with his bare hands to pull away small chunks of concrete said to date back to work done in 1917.

“We have a responsibility to leave the building safe and secure,” declared one official. “You don’t know what you’re going to find until you go in there.”

Workers have been finding additional flaws in City Hall since the project to restore it was announced in 2007, when the cost was estimated at $60 million.

A decision to add a sub-basement for new electrical and mechanical systems, as well as sprinklers, a new elevator, an additional staircase and a new fire-alarm system inflated the figure to $90 million as of November 2009.

Work began in earnest in mid-2010, but was temporarily halted when workers unearthed on the north side of the building a Revolutionary War-era settlement complete with relics.

Nature intervened last year when an earthquake shook the entire building and cracked a section of the front portico. That damage alone is going to cost $1 million to fix.

By last year, the price of restoring the structure to its 1812 brilliance had swelled to $123.8 million.

Officials said the investment in the roof makes sense, since the next mayor would be stuck with a bill even higher than $150 million if the project were delayed.

“As anyone who has done repairs knows, fixing a roof is a costly but necessary endeavor,” said mayoral spokeswoman Julie Wood.